In an electric linear motion actuator having an electric motor as a drive source, the rotational movement of a rotor shaft of the electric motor is converted to a linear movement of an axially movable driven member by a motion converting mechanism.
A ball screw mechanism and a ball ramp mechanism are known for the motion converting mechanism adopted in the electric linear motion actuator. Although such motion converting mechanism has a force multiplying function to a certain extent, a large force multiplying function required in the electric disc brake system, and the like cannot be ensured.
Thus, in the electric linear motion actuator adopting the motion converting mechanism described above, the drive force is increased by separately incorporating a speed-reducing mechanism such as a planetary gear mechanism, whereby the configuration becomes complex and the electric linear motion actuator enlarges by the incorporation of the speed-reducing mechanism.
In order to solve such problems, the applicant of the present application has already proposed, in the below-identified patent document 1 and patent document 2, an electric linear motion actuator that can ensure a large force multiplying function without incorporating the speed-reducing mechanism and that is suited for use in the electric disc brake system in which a linear stroke is relatively small.
In the electric linear motion actuator described in patent document 1 and patent document 2, planetary rollers are mounted between a rotation shaft, which is rotation driven by an electric motor, and an axially movable outer ring member. When the rotation shaft is rotated, the planetary rollers are revolved while spinning by frictional contact with the rotation shaft, and the outer ring member is axially moved due to engagement of a helical rib formed on the radially inner surface of the outer ring member in a helical groove or circumferential grooves formed in the radially outer surface of each planetary roller.
In the electric disc brake system adopting the electric linear motion actuator described in patent document 1 and patent document 2, only a service brake function that controls the braking force in accordance with the operation of the brake pedal of the driver is provided, and hence the electric motor needs to be kept energized to maintain the braking force at the time of parking, which is extremely disadvantageous in terms of power consumption.
A great effect can be obtained in resolving the disadvantages described above by adding a parking brake locking mechanism by a solenoid and regulating the rotation of the rotor of the electric motor in the brake-releasing direction by the actuation of the parking brake locking mechanism as in an electric brake system described in patent document 3.
However, in the electric brake system described in patent document 3, although the parking brake locking mechanism is arranged, such parking brake locking mechanism is arranged at a periphery of the rotor and hence the radial dimension becomes large and the parking brake locking mechanism may interfere with the wheel when assembled to a vehicle. Furthermore, since the parking brake locking mechanism is arranged at the periphery of the rotor, the weight on the electric motor side becomes heavy thus causing the weight balance to degrade. Such degradation in the weight balance adversely affects the contact of the brake pad at the time of braking, whereby the braking becomes unstable and a brake noise may generate.
The electric brake system described in patent document 4, a locking mechanism for locking and unlocking the rotation of the rotor shaft of the electric motor includes a plurality of locking portions arranged in the circumferential direction of a side surface of one of a plurality of gears forming a gear reduction mechanism, a locking pin that can move forward and backward with respect to the locking portions, and a pin driving actuator that moves the locking pin forward and backward. The locking pin and the pin driving actuator are incorporated between the electric motor and a housing that houses a slide member and a rotation/linear movement converting mechanism, whereby the electric disc brake system that is small and compact and that excels in weight balance is realized.